Infrared transient absorption and excited-states of excitons and biexcitons confined in CuCl quantum dots

Infrared transient absorption and excited-states of excitons and biexcitons confined in CuCl quantum dots

ARTICLE IN PRESS Journal of Luminescence 108 (2004) 371–374 Infrared transient absorption and excited-states of excitons and biexcitons confined in C...

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Journal of Luminescence 108 (2004) 371–374

Infrared transient absorption and excited-states of excitons and biexcitons confined in CuCl quantum dots K. Miyajimaa,*, K. Edamatsub, T. Itoha a

Division of Materials Physics, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan b Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Abstract Mid-infrared picosecond transient absorption has revealed a new aspect of the excited states of confined biexcitons in CuCl quantum dots embedded in a NaCl matrix. The transient absorption exhibits two-decay components that originate from confined excitons and biexcitons. The attribution of the fast-decay component to the confined biexcitons has been confirmed by measurement by a two-photon resonant excitation process. The transient-absorption spectrum of biexcitons indicates the existence of a stable biexciton excited state in the quantum dots, in contrast to the unstable excited state in the bulk crystal. r 2004 Elsevier B.V. All rights reserved. PACS: 73.21.La; 78.67.Hc Keywords: CuCl; Quantum dot; Transient absorption; Biexciton

1. Introduction In recent years, optical investigations of semiconductor nanocrystals or quantum dots (QDs) have been performed extensively. The electronic structure in the QDs shows atomic-like discrete energy levels by three-dimensional confinement of electrons and holes. Previous investigations have been concentrated on their lowest-state properties. However, the optical nonlinearity associated with the transitions between sublevels of the electron– *Corresponding author. Tel.: +81-6-6850-6508; fax: +81-66850-6507. E-mail address: [email protected] (K. Miyajima).

hole pairs (excitons) are expected to be available for novel infrared devices. Therefore, it is important to study the optical properties, not only of the lowest state, but also of the higher excited states of confined excitons or multi-excitons. Up to now, there have been a number of reports on the excited-exciton states confined in semiconductor QDs by photo-induced transient absorption [1–3]. However, no study on the excited states of confined biexcitons has been reported, although large optical nonlinearity is expected by large correlations between two excitons. In addition, quantum confinement effects in the relative motions between two excitons are thought to induce specific transition dynamics that would not appear in bulk crystals.

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.078

ARTICLE IN PRESS K. Miyajima et al. / Journal of Luminescence 108 (2004) 371–374

CuCl QDs show the typical characteristics of the weak confinement regime, in which the center-ofmass motion of the exciton is quantized because of the small exciton Bohr radius (aB=0.7 nm). In addition, the concept of a biexciton as a quasiparticle is effective in large quantum dots (R>3 nm) [4]. For this material, we found the appearance of infrared transient absorption (IRTA) that was attributed to the biexciton state [5]. In this paper, we have measured the photoluminescence (PL) spectra and the IRTA under two-photon resonant excitation of the confined biexcitons in order to confirm the origin of IRTA as due to biexcitons. The PL and IRTA intensities show a similar polarization dependence on the pump beam, following the selection rules for the biexciton state [6,7]. These results support the interpretation of the IRTA as caused by biexcitons confined in semiconductor QDs

2. Experimental CuCl QDs embedded in a NaCl matrix were prepared by the transverse Bridgman method followed by successive annealing treatments [8]. We performed IRTA measurements by pump-andprobe spectroscopy with a dual optical parametric amplifier (OPA) system pumped by an amplified mode-locked Ti:sapphire laser (wavelength 800 nm, repetition rate 1 kHz, pulse width B2 ps). The pump pulse was obtained by fourthharmonic generation of the signal beam of one OPA and the wavelength was tuned to sizeselectively excite the excitons or biexcitons confined in the QDs. Infrared probe pulses were generated from the idler beam or by differencefrequency-mixing of the signal and idler beams of another OPA. The probe-pulse intensity was reduced by neutral-density filters to B0.1 mJ/pulse which is much lower than the pump intensity (maximum 8 mJ/pulse). The probe-pulse intensity was detected by a liquid nitrogen-cooled HgCdTe photodiode, and the differential-absorption signal was detected by a lock-in amplifier set at the chopping frequency of the pump light. The sample temperature was kept at 70 K during the measurement.

3. Results and discussion Fig. 1 shows a typical temporal profile of the IRTA measured at the probe-photon energy of 250 meV with the excitation light at 3.227 eV which corresponds to the resonant excitation of confined excitons in the QD with effective radius (a) of B4.2 nm. The IRTA exhibits two decay components. The fast and slow decay times are B56715 and B4907290 ps, respectively. These decay times are almost the same as the previously-reported lifetimes of the biexciton (B65 ps) and exciton (B380 ps) in a CuCl QD of similar size, respectively [9]. From this result, it is proposed that the fast and slow decay components are induced by the confined biexciton and exciton states, respectively. In order to confirm that the IRTA is caused by the biexciton, we examined the two-photon resonant excitation of the confined biexciton. Since the lowest biexciton state with total angular momentum J=0 is composed of two excitons with the opposite angular momenta ( j=71), the lowest biexciton state would not be optically allowed for co-circularly-polarized excitation light [6,7]. Fig. 2 shows the PL spectra and IRTA temporal profiles under excitation with a photon energy of 3.204 eV, which is lower than the resonant energy of free excitons in the bulk crystals. We clearly observed 0.1 τ1~56ps





1E-3 0





Time (ps) Fig. 1. Temporal profile of the IRTA measured at a probephoton energy of 250 meV. The excitation was at 3.227 eV, which is resonant with the exciton confined in the QD (effective radius: a=4.2 nm). The solid curve is fitted to the data assuming two decay components (56 and 490 ps).

ARTICLE IN PRESS K. Miyajima et al. / Journal of Luminescence 108 (2004) 371–374

PL Intensity (a.u.)

circular exciton


x10 3.12




0.03 ∆O.D.






Photon Energy (eV)

linear circular



0.00 0 (b)


100 150 Time (ps)



Fig. 2. PL spectra (a) and temporal profile of the IRTA (b) under two-photon resonant excitation of confined biexcitons with linearly (solid line) and circularly (dashed line) polarized pump-light excitation.

the PL bands of the biexcitons (B3.18 eV) and of excitons (3.235 eV) created by the cascade relaxation process of the biexciton. In addition, those PL intensities decreased under circularly-polarized excitation compared to linearly polarized excitation. These results indicate that the biexcitons are created directly via two-photon resonant excitation process and the selection rules were effective, although not perfect, unlike the case of the bulk crystals. The IRTA of the fast component also decreased for circularly-polarized light, which is similar to the case of biexciton PL spectra. As a result, we concluded that the fast-decay component of IRTA originates from the decay of the lowest biexciton state. More detailed results and discussion about the two-photon resonant excita-

tion process of the confined biexcitons will be reported in the near future. Fig. 3 presents the IRTA spectra originating from the biexciton and the exciton under resonant excitation of confined excitons in the dots (a=4.2 nm). The transient-absorption spectrum of the biexcitons has a main peak at 225 meV with a tail toward the higher energy side. On the contrary, the transient-absorption spectrum of the excitons has a broad band. However, any fine structure could not be detected because of the insufficient signal to noise ratio. Therefore, we focus on the excited biexciton states observed in the IRTA spectrum. Up to now, there has been a report on the theoretical calculation of the IRTA for the confined excitons [10]. However, the authors do not know of any report for the confined biexcitons. Experimentally, the IRTA spectral shape of the biexcitons was quite similar to that of the excitons predicted by the theoretical calculation [5]. Moreover, the obtained peak energy of the biexcitons eventually almost coincides with the transition energy of the Rydberg 1s– 2p state of the confined excitons [5]. In order to explain these results, we assume the following energy diagram for the exciton–biexciton system as shown in Fig. 4. The lowest state of the biexciton is composed of two 1s excitons. The excited–biexciton state, which is observed in the IRTA spectrum, is composed of one 1s lowest exciton and one excited exciton (e.g. 2p state). In general, such an excited-state would not be stable 0.15 biexciton exciton

0.10 ∆O.D.




0.00 100






Photon Energy (meV) Fig. 3. The IRTA spectra due to the biexciton (closed circles) and the exciton (open circles).

ARTICLE IN PRESS K. Miyajima et al. / Journal of Luminescence 108 (2004) 371–374


excited biexciton 2 excitons excited exciton



IRTA exciton

pump ground

resolution. We have observed two decay components in the IRTA temporal profile. By the measurement of two-photon resonant excitation of confined biexcitons, these decay components are confirmed to be attributable to confined biexcitons and excitons. From the analysis of IRTA spectrum, it is proposed that the excited biexciton states may not be much different from those states composed of one exciton in the lowest state and the other in an excited.

pump ground

Fig 4. Energy diagram of observed transient absorption originating from a confined exciton (left) and a biexciton (right).

in bulk crystals and the biexciton would dissociate into two excitons since the transition energy to the excited biexciton states is considerably larger than the biexciton binding energy (B50 meV). With three-dimensional confinement as in the case of QDs, however, the two excitons cannot dissociate from each other so that the excited biexciton state turns out to be stable. Under this assumption, it is reasonable to think that the observed IRTA spectrum of the biexciton is quite similar to that of the exciton, which is dominated mainly by the transition from 1s to 2p Rydberg states.

4. Conclusion We have measured the IRTA for CuCl quantum dots embedded in a NaCl matrix by pump-andprobe spectroscopy with picosecond temporal

Acknowledgements This work was supported by CREST (Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology Corporation).

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